Synthesis, crystal structure and characterization of novel open framework CHA-type aluminophosphate involving a...

A highly crystalline new precursor of CHA-type zeolite AlPO₄-34 was obtained by using an aluminophosphate gel with a chiral diamine [(S)-(+)-1-(2-pyrrolidinylmethyl)pyrrolidine; C₉H₁₈N₂] as an organic structure directing agent under hydrothermal synthesis conditions. This precursor (called GAM-1) was changed into the zeolite AlPO₄-34 which had a high porosity (the Brunauer-Emmett-Teller (BET) surface area is approximately 700 m³ g^-1) using calcination. The GAM-1 obtained was characterized by various measurements, e.g., powder X-ray diffraction, scanning electron microscopy, and solid-state nuclear magnetic resonance spectroscopy, and so on. Structure determination from powder diffraction data revealed that the new precursor GAM-1 has triclinic symmetry [space group P1[combining macron], a = 9.16535(11) Å, b = 9.23042(11) Å, c = 9.29228(11) Å, α = 79.8243(7)°, β = 87.4593(7)°, γ = 86.5365(7)°] and the chemical formula was estimated to be: |Al₆P₆O₂₄H₂F₂|(C₉H₁₈N₂)·2.5(H₂O). It also revealed that a two edge sharing AlO₄F₂ octahedron with an [triple bond, length as m-dash]Al-F-Al[triple bond, length as m-dash] bridge was included in the framework. GAM-1 was transformed into AlPO₄-34 with rhombohedral symmetry (R3[combining macron]) by elevating temperature to over 400 °C. At high temperatures, AlO₄F₂ octahedron connectivity was changed into an AlO₄ tetrahedron. The crystal structure of the dehydrated AlPO₄-34 changed back to a triclinic symmetry (P1) model again after rehydration in the atmosphere.